Transparent Lateral Boundary Conditions for HIRLAM. The final
shallow water tests and a first multi-level implementation.
A McDonald
Met Éireann
Dublin, Ireland.
(Manuscript received 23 October 2003, in final form 2 February 2004.)
Abstract
First, some remaining issues concerning transparent boundary conditions
for the shallow water equations are addressed. (a) What is the correct
discretization when the advection velocity is greater than the gravity
wave speed? (b) Will the introduction of orographic features on the
boundary create instabilities? (c) Can horizontal diffusion be
incorporated without generating numerical boundary layers? Second,
having answered these questions satisfactorily, the simplest transparent
boundary conditions, characteristic boundaries are incorporated into a
multi-level model. A 48h forecast is found to be stable and accurate.